Many operators of motor vehicles employ receiving apparatus for detecting incident ranging signals, i.e., signals transmitted from radar transmitters or laser light emitters employed by police authorities to measure the velocity of the vehicle. Typically, commercially available detectors of ranging signals and laser light used in monitoring vehicle speed, conventionally generically called radar detectors, employ an appropriate sensor and a demodulating circuit connected to the sensor. The sensor may include a conventional antenna for detecting electromagnetic signals in a particular frequency range. The sensor may also include a light sensor, particularly an infrared or laser light sensor, detecting light signals that are sometimes now employed in monitoring vehicle speed. In conventional radar detectors and laser light detectors, upon detection of a signal of interest incident on a vehicle, an alarm is actuated. Typically, the alarm may be a light or an audio signal or both light and audio to obtain the attention of a vehicle occupant promptly.
Vehicular detectors of emergency vehicles, such as ambulances or police vehicles, that are responding to emergencies and provide warnings with flashing lights and/or sirens are also known. An example of such an emergency vehicle alarm system is disclosed in U.S. Pat. No. 5,495,243. The alarm system described in that patent detects light signals, not radio signals, produced by the emergency vehicle lights. Thus, the alarm system is desirably limited to the detection of nearby emergency vehicles, i.e., emergency vehicles within a "line-of-sight". Since radio signals can penetrate stationary objects, a similar detector using radio signals may detect emergency vehicles that are unlikely to be directly encountered. When using light detection only, emergency vehicles that are far away from or not likely to interfere with the movement of the vehicle including the emergency vehicle alarm system are not detected.
Many emergency vehicles are equipped with warning lights that emit both visible and infrared light. The light may be modulated at a relatively low frequency but at a frequency high enough to be undetectable to the eye. That modulation can be employed, with appropriate detectors, for switching traffic signals so that the emergency vehicle can proceed as quickly as possible through intersections by switching all signals to green for the direction of travel of the emergency vehicle and to red for all other directions of vehicle travel. For example, in one known apparatus, the warning light is modulated at a frequency of 14 Hz, although other frequencies, such as up to about 30 Hz, can easily be employed in modulating, i.e., strobing of, the emergency warning light.